1. Technical Field
The invention relates to control systems for electromagnetic impact rappers for electrostatic precipitators and in particular to the regulation of the electrical power or energy delivered to the magnetic rappers so as to maintain consistent rapping energy output over a wide variation of ambient operating temperatures about the rappers.
2. Background Information
Electromagnetic impact rappers are widely used for cleaning the discharge wires and collecting plates of electrostatic precipitators which remove or recover particulate matter from moving gas streams in industries such as electrical power generation, steel making, cement plants, paper mills and the like. These rappers usually are installed within a weather resistant enclosure or are exposed to the outside ambient conditions. In both cases, rapper coil operating temperature is uncontrolled. Solar heating, combined with day to night ambient temperature changes, plus seasonal temperature changes, can easily cause a 40.degree. to 50.degree. centigrade change in rapper coil operating temperature with a resultant 16% to 20% change in coil resistance. Although the duty cycle of the rappers is low enough that the effects of self heating of the coil due to power dissipation within the coil is negligible, actual testing of usual electromagnetic impact rappers indicate that a 25% to 50% change in energy output over a 50.degree. centigrade temperature excursion will occur. This relatively large variation in rapper output energy can cause deteriorated precipitator performance and precipitator structural damage.
Common electromagnetic rapper control systems and known methods energize the rapper coil with a preselected voltage applied for a preselected time. As the rapper coil resistance changes with changing operating temperatures, the current through the rapper coil also will change. This changes the strength of the magnetic field generated by the rapper coil, the velocity of the rapper slug as it is lifted against gravity or against the spring, and thus changes the potential energy stored in the rapper slug or slug and spring combination for subsequent transfer to the precipitator discharge electrode or collecting plate from which the collected matter is to be removed, and for certain types of rappers, it changes the magnetic force which directly causes the impact in combination with a return spring.
Various known prior art electromagnetic rappers and control systems are shown in the following patents.
U.S. Pat. No. 3,504,480 discloses a control system for a series of electrostatic precipitator rappers of the type with which the present control system may be utilized. Various feedback circuits are used in this system to regulate the firing angle of SCR devices to change the power supplied to the rappers. The feedback signals are from operating parameters such as the precipitator sparking rate, precipitator current and for changes in line voltage.
U.S. Pat. No. 4,285,024 shows another type of electrostatic precipitator rapper control system which includes means for sensing currents applied to each rapper coil during a controlled energy pulse. The resultant sensed current is integrated with respect to time over the period of the pulse of controlled energy and the results of the integration will indicate plunger displacement.
U.S. Pat. No. 4,693,732 discloses a rapper that can be adjusted to compensate for various factors including thermal expansion of the rapper mechanism. However, the adjustment is entirely manual and is accomplished by movement of lock nuts along a threaded shaft containing coil springs.
However, none of these known electrostatic precipitator control systems show the sensing of the ambient temperature about the rapper nor provide means for the subsequent compensation thereof, to effect the energy delivered by the rapper to the electrostatic precipitator collecting plates and discharge electrodes as in the present invention.
U.S. Pat. No. 4,456,898 discloses a type of ambient temperature magnetic control device completely unrelated to the electrostatic precipitator art.
U.S. Pat. No. 4,459,574 discloses another control circuit using electromagnetic force generation which provides temperature compensation of the coil current in a camera for controlling the shutter blades.
U.S. Pat. No. 4,793,372 discloses a solenoid valve in which the resistance of the coil changes with an increase or decrease in temperature to vary the amount of flux generated. Again, such a device is not used or associated in any manner with an electrostatic precipitator rapper as is the present invention.
Thus, the need exists for an improved rapper control and energization system that will maintain relatively constant rapper energy delivery to an electrostatic precipitator rapper over a wide variation in ambient operating temperatures about the rapper and electrostatic precipitator.